OSI and TCP Model Overview
The OSI. (Open Source Initiative) model is a way of thinking of networks that lets you divide it into layers, and assist with trouble shooting as it lets you identify where the issue is occurring. At a high level, networking is a way of sending data from one location to another. Layers break it down so you understand the different parts of the networking process, but they all work together simultaneously. The layers are as follows:
- Layer 1: Physical Layer
- Layer 2: Data Layer
- Layer 3: Network Layer
- Layer 4: Transport Layer
- Layer 5: Session Layer
- Layer 6: Presentation Layer
- Layer 7: Application Layer
These layers are easily remembered with the following mnemonic:
Please Do Not Throw Sausage Pizza Away
Just as the shipping of a physical package has different components and processes, the same goes for networking. It is important to remember that data goes up and down these layers. When computers transmits data, the data goes from layers 1 to 7 (Encapsulation) and when computers receive data (Decapsulation), they go from layers 7 to 1.
TCP/IP Model
The TCP/IP model is a more broad view of the OSI model, but the TCP/IP model combines OSI layers, as you can see in the diagram below. All layers function exactly the same as in the OSI. The only major difference is that the TCP/IP model combines OSI’s session, presentation and application layers.
OSI Model Layer 1: Physical Layer
This is the physical and electrical stage and is the first point in the transfer of data between locations. This is all about cables and bits – includes electrical impulses, wireless data. This is where we convert signals to data and vice versa.
Packets and data get converted to electrical signals in order to send them over networks. Elements of this layer includes:
- Pin outs on connectors;
- Voltages across cables;
- Cable specifications; and,
- Networks interface cards (including ports)
OSI Model Layer 2: Data Link Layer
This layer provides access to the computer. It involves packaging electrical impulse from Layer 1 into FRAMES, segments of data. This includes:
- Access: Transfer from point to point;
- MAC address: Media Access Control addresses that are unique and hard coded into every device world wide. These are not globally routable Encoded into network interface card;
- Data that is sent from a single computer to another computer on the same network, it is a Layer 2 connection as data is addressed by MAC address. A hub would be a layer 2 device as it doesn’t do any routing.
OSI Model Layer 3: Network Layer
Also know as the IP layer and is responsible for facilitating the transfer across different networks. Is responsible for translating logical to physical addresses.
Performs:
- Network routing. Routers work here.
- Fragmentation/reassembly of packets. Takes frames from layer 2 and turns them into packets. Packets are what gets routed by routers.
You can’t transfer data globally or to subnets via MAC addresses due to the way networks are set up. Routers need IP addresses to route traffic. Routers have two IP address, an internal one for the network and external one for routing traffic externally. Internally, routers route via Layer 2 and send data to specific MAC addresses.
OSI Model Layer 4: Transport Layer
- Provides management and control of data packets;
- Transfers data;
- Splits communicate into packages with either:
- TCP: Transmission Control Protocol – provide delivery verification.
- UDP: User Datagram Protocol – provides no delivery verification. This is used for streaming data like videos. This data already uses a lot of bandwidth, so we don’t want to use more bandwidth on verification as it will create lag.
Do not confuse with Layer 5, the Session Layer.
OSI Model Layer 5: Session Layer
Provides traffic control between connections. This involves:
- Establishing, managing and terminating connection;
- Regulates when computers can send data and how much they send;
- Coordinates conversations.
When thinking about different layers before it, they simply send/receive data. This is the first layer that controls the sequence of events, which is needed before computers can actually communicate with one another. It establishes a “Handshake” with another computer.
OSI Model Layer 6: Presentation Layer
- Takes the packages of data and translates them into a form where the application layer can understand them;
- Provides independence from difference in data representation in most cases (you can’t translate data formats at this layers);
- Encrypts and decrypts data.
OSI Model Layer 7 Application
- Operates on your computer;
- Network access layer for applications;
- Enables application to access network;
- Identifies communication patterns;
- Determines resource availability; and.
- Synchronizes communications with protocols includes:
- HTTP
- SMTP
- FTP
